Artificial kidney systems generally consist of several major components, some of which are used only once and others of which are used repeatedly. Typically, the artificial kidney itself is used only once and then discarded, together with all related tubing, needle assemblies, integral tubing clamps, injection ports and the like. Likewise, after passing through the artificial kidney, the spent dialysate solution is discharged into a suitable waste disposal system. In contrast, the dialysis machine, blood pump and anticoagulent injection pump are reused, each being appropriately cleaned between treatments. In addition to these patient-specific components, the dialysis treatment center typically includes a suitably-sized water conditioning system, usually of the reverse osmosis (RO) or deionization/reionization type. Convenient access must also be provided to a source of dialysate concentrate appropriate for the machine and the patient. Depending upon the design of the dialysis machine, provision may also be necessary for returning excess dialysate concentrate to the supply source.
Although the exact sequence varies with the particular type of dialysis machine, preparation for a typical treatment usually begins by flushing an appropriate sanitizing solution such as formaldehyde through all of the internal fluid passages of the machine. At the end of a suitable sanitizing time period, the sterilant is then purged from the machine, typically using the conditioned water. The machine can then be coupled to the source of dialysate concentrate and priming initiated.
During the priming operation, the dialysis machine proceeds to produce dialysate in accordance with several parameters set by the operator via input switches, dials and the like. Typically, these parameters include the temperature at which the dialysate is to be delivered to the artificial kidney, the rate at which the dialysate is to be delivered, and the pressure at which the dialysate is to be delivered. In the priming mode, however, the dialysate being produced is either internally or externally recirculated so as to bypass the artificial kidney. Once the dialysis machine determines that the dialysate is being produced at the selected temperature, flow rate and pressure, the operator may be notified that dialysis may be commenced.
While the dialysis machine is stabilizing, the operator may be preparing the patient by coupling the blood tubing assembly between the patient and the artificial kidney, with an appropriate portion thereof installed in the associated blood pump. Typically, an infusion pump is also coupled to the blood tubing and adjusted to provide a controlled rate of injection of a suitable anticoagulent such as heparin. In most systems, venous and arterial pressure monitors are also coupled to the blood tubing assemble so that dangerous pressure levels may be detected before the patient is injured.
Once the machine is primed and the patient fully prepared, the operator will enable the dialysis machine to circulate the conditioned dialysate through the artificial kidney, while maintaining a selected negative transmembrane pressure within the artificial kidney so that excess fluids may be removed from the patient's blood. Typically, while operating in this dialyze mode, the dialysis machine continues to monitor the temperature, flow rate, concentration and pressure of the dialysate delivered to the artificial kidney. If any of these parameters deviates beyond the capability of the machine to correct, the dialysis machine will sound an alarm and immediately terminate delivery of the dialysate. In the event of such a shutdown, operator intervention is required before the dialysis machine will resume delivery of the dialysate.
One of the more automatic and in genereall safest of the type of dialysis machine just described is that shown and described in U.S. Pat. No. 4,153,554. However, the performance of the latter machine was limited in several areas. One primary source of limitation was the response delay inherent in the several servoloops within the control system. For example, due to the large size of the water heater used to heat the conditioned supply water to the desired delivery temperature, the lag time between a sensed temperature deviation and actual sensed response was significant. This problem was exacerabated by variations in the temperature of the supply water as well as changes in the flow rate of the water through the heating chamber. In general, the reservoir approach used to compensate for this problem was only partially successful.
One other major source of performance limitation in this earlier system was the type of sensors used to sense such parameters as the temperature of the conditioned water and the conductivity of the dialysate. Since these sensors were electrically powered, variations in the supply voltage could result in different readings although the sensed parameter in fact had not changed. However, because the control circuit was unable to distinguish between the causes of the sensed deviations, corrective actions were set in motion which were not only unnecessary but also disruptive in and of themselves. Again, the averaging approach used therein to compensate for instantaneous variations in supply voltage were only partially successful, particularly since relatively long term voltage drift is not at all uncommon in many areas. In addition, this introduced further delay in the response loop.
Another aspect of this same problem related to the variation of the pump speeds due to variations in supply voltage. Even though the dialysate concentrate supply side of the manifold system was ratioed so that both the water and the dialysate concentrate pumps could be driven at the same speed to maintain consistent concentration, the overall dialysate delivery rate still varied as a result of the variation in operating speed of both pumps.